1. Field of the Invention
The present invention relates to a device for controlling intake air quantity in which a throttle valve is rotated by a brushless motor to control the quantity of intake air to be supplied to an engine.
2. Description of the Related Art
In automobiles, a throttle valve is provided in the intake air passageway of the engine. This throttle valve is opened and closed in conjunction with the operation of an accelerator pedal by the operator. Therefore, the quantity of intake air to be supplied to the engine is controlled according to an operation quantity (a quantity of depression) of the accelerator pedal.
Such control of intake air quantity is accomplished by coupling the throttle valve and the accelerator pedal through a mechanical coupling means such as a link or a wire. However, since such mechanical coupling means limits the positional relation of the accelerator pedal and the throttle valve, the problem of a reduced degree of freedom for mounting positions in an automobile has existed.
Further, in recent years, controlling the throttle valve independently of the operation of the accelerator pedal by an operator has become necessary for controlling cruise control devices or traction control devices. Therefore a method of electrically coupling a throttle valve and a motor has been proposed. For example, the control of a throttle valve as disclosed in Japanese Patent Laidopen No. HEI 1-315641 is well-known. If a motor with a brush commutator is used, the pressure applied to the motor by the brush commutator will cause hysteresis torque to be produced in the positive-rotation(forward) and negative-rotation(reverse) directions of a rotor and therefore position control will become difficult. For this reason, in the above-described reference, a brushless motor is used to control the throttle valve.
Further, in the control of a throttle valve disclosed in Japanese Patent Laidopen No. HEI 5-240070, the rotor of a brushless motor and the rotational shaft of a throttle valve are coupled through a speed reducer in the form of gears to control the throttle valve with a high degree of accuracy.
Also, in order to switch the stator windings (hereinafter referred to as phases) of a brushless motor, a counter electromotive force detector for detecting a counter electromotive force at a phase and a current switch detector are provided, thereby requiring a rotation detector which is expensive and highly precise.
However, in devices for controlling the quantity of intake air which perform the above-described conventional control a throttle valve, there are the following two problems.
First, in the device disclosed in the above Japanese Patent Laidopen No. HEI 1-315641 and Japanese Patent Laidopen No. HEI 5-240070, even if a brushless motor were used, the hysteresis torque with respect to rotational direction will not be eliminated because of the existence of bearings supporting the rotational shaft of a throttle valve, a sealing member preventing infiltration of foreign substances from the outside, a speed reducer and gears, a return spring for closing a throttle valve when an abnormality occurs, and frictional resistance between the sliding parts other than the brush commutator, such as the sliding parts inside the brushless motor.
With respect to this point, a measured example of the driving torque characteristics of a throttle actuator constructed with a brushless motor will be described with reference to FIG. 12. In the figure, the abscissa represents throttle opening ratio and the ordinate represents driving torque. When the throttle valve is driven in its opening direction the driving torque of the throttle actuator changes along the upper characteristic and when the throttle valve is driven in its closing direction it moves along the lower characteristic. The difference in the driving torques generated by the different opening and closing directions is called hysteresis torque. Note that the driving torque characteristic rises to the right with the same slope as the spring constant of a return spring.
The driving torque is proportional to a phase current Is (=PWM duty), so the ordinate can be replaced with the phase current Is. When the current degree of throttle opening is shifted to the next degree of throttle opening by the position feedback control of the throttle valve using a motor, the phase current Is is increased or decreased from its current value to increase or decrease the driving torque of the motor. However, since the throttle valve will not move in the hysteresis torque range even if the phase current is changed, response properties will be hindered. This problem is particularly evident in the fine control of throttle opening where the degree of movement of the throttle opening (quantity of change in the phase current) is small.
Second, there is the problem that the drive of a brushless motor, when, based on the outputs of a counter electromotive force detector and a current change detector, a certain current conducting phase is switched to the next current conducting phase, the current abruptly changes. Therefore, when there is a delay between the change in magnetic flux applied to the phase and the above-described detector output, the torque generated by the motor becomes discontinuous, as will be described later. As a result, the degree of throttle opening changes abruptly.
The principles of the above problem will be described with reference FIG. 13.
As is apparent from FIG. 13, the phase current to be conducted through each winding (phase) is conducted based on the detector output (for example, a detector for generating an output signal every 30 degrees) corresponding to the magnetic flux density where each phase crosses, as a rotor is rotated. For example, if it is determined by the detector output that an A-phase winding has crossed with a magnetic flux density of predetermined size, a current will be conducted to the A-phase winding. Next, when the motor shaft is rotated and the magnetic flux density moves to a C-phase winding, a current will be conducted to the C-phase winding in the same manner. Next, a current is likewise conducted to a B-phase winding. That is, the current conducting phases are cyclically switched while they are overlapped by 30 degrees each. By this type of ideal switching of phases where the phases are linked with the magnetic flux density change where the phases cross in the way described above, a motor shaft torque (indicated by the solid line in the figure) wherein the torques generated in the phases are continuously linked is obtained.
However, if it is now assumed that the output signal of the detector lags in the direction indicated by the arrow shown in FIG. 13, the conducting angle of the A-phase will increase and the conducting angle of the B-phase will decrease, so the motor shaft torque will be discontinuous (shown by the broken line). Since the motor shaft torque abruptly increases at this point of discontinuity, the degree of throttle opening changes abruptly. On the other hand, as is clear from the description above, it is necessary that the mounting position of the detector which acts as a reference for switching current conducting phases be highly accurate with respect to the position of the stator winding. However, due to variations in production etc. the problem that the above-described lag cannot be completely eliminated remains.
In order to overcome this problem, it is conceivable to adopt a three-phase conducting system in which a current is independently supplied in the form of a sine wave for each of the A-phase, B-phase, and C-phase, but such a system would have the problem that it requires a detector for accurately measuring the angle of rotation of a motor rotor.